A) The establishment of systems is envisaged to permit, first, the intracellular generation of replicating and transcribing measles virus (MV) subgenomic ribonucleoprotein (RNP) from cDNA plasmid vectors specifying artificial subgenomic MV RNA. Different self-cleaving transcripts have been devised, all containing the authentic 5' and 3' genomic or antigenomic termini, the leader and trailer regions as well as the noncoding regions of the first and last gene, but distinguished by their different number of either MV-specific of heterologous (reporter protein or drug resistance) reading frames surrounded-by precise MV noncoding and/or intergenic regions. Three generations of virus-free helper cells are designed to provide the MV proteins required for MV genome encapsidation, transcription and replication as well as the T7 polymerase, to transcribe transfected subgenomic cDNA plasmids. Second, the generation of infectious MV is envisaged by substituting the subgenomic by full length genomic vector. These reverse genetics systems will allow to develop fully characterized live attenuated vaccine strains from existing strains by judicious introduction of any desired modification to either follow the evolution of circulating MV strains or to make use of traits identified enhancing antigenicity or stabilizing attenuation. In addition, introduction of sequences encoding antigens of heterologous pathogens into the MV genomic vectors is proposed. These DNA segments will be inserted by two different approaches into the MV genomic context in such a way that interference with the replication and transcription of rescued hybrid MV will be minimized. It is hoped that such modified MVs, which should be as safe as the original vaccine strains and much cheaper to produce than subunit vaccines, would mediate as long-lasting additional immunity against a foreign pathogen (such as hepatitis B virus) as that raised against MV. B) The generation of transgenic mice supporting normal MV infection and replication is aimed at by knock-out replacement of mouse genes by the homologous human genes encoding MV receptor(s) being characterized in several laboratories. Although these transgenics will probably not mimic the human disease equally well as primate animals, e.g. cynomolgus monkeys, they should provide an economically reproducing animal model useful for the study of various aspects of MV biology such as immunosuppression and for the initial testing of newly developed MV vaccines.